1. Field of the Invention
The present invention relates to an image forming device, such as a copy machine, a printer, and a facsimile machine, and a developing unit included in the image forming device.
2. Description of the Related Art
A well-known electrophotographic image forming device uses a photoconductive drum as an image carrier. By charging and exposing a surface of the photoconductive drum, an electrostatic latent image is formed on the surface of the photoconductive drum. Toner adheres onto the electrostatic latent image, and a visible image is formed. This visible image is transferred onto paper. In such an image forming device, components are unitized according to necessity, and this unit is removably inserted in the image forming device. When a replacement timing of the components arrives, the unit is replaced as appropriate. In particular, a developing unit containing the toner, which is consumable, is replaced when a remaining amount of the toner becomes low. The photoconductive drum and the developing unit may be integrally provided as one unit.
In order to accurately grasp the replacement timing of such a unit to be replaced, information indicating an arrival of the replacement timing is loaded in the unit. For example, according to a first conventional device, a unit includes a nonvolatile memory which stores identification data indicating a serial number of the unit and count data indicating a cumulative used time. Accordingly, the unit inserted in a main body of the image forming device can be distinguished. Each unit includes the memory which stores the used time associated with the identification data. According to a second conventional device, a cumulative number of images is stored in a main body memory of a printer main body and a memory of a cartridge. When the cartridge is inserted in the printer main body, the cumulative number of images stored in the memory of the cartridge is read and stored into the main body memory of the printer main body. A number of sheets on which an image forming process has been performed is added to the cumulative number of images stored in the main body memory. After the image forming process has been completed, the cumulative number of images stored in the main body memory is also stored into the memory of the cartridge.
In the above-described image forming device, although a plurality of types of replaceable developing units are used, there is a disadvantage that a developing bias applied to a developing roller of the developing unit cannot be controlled accurately. The toner accumulated in the developing unit is charged while being agitated, and the toner adheres onto the surface of the photoconductive drum by an electric potential difference between the developing roller and the photoconductive drum. If the developing bias applied to the developing roller is not set according to a charged amount of the toner, a normal developing process cannot be carried out. The charged amount of the toner tends to decrease accompanying a usage performance of the developing unit (for example, a cumulative used time and a cumulative number of sheets on which the image forming process has been performed by the developing unit). Therefore, the developing bias is also required to be changed accompanying this tendency. However, when the developing unit is replaced, a usage performance of the image forming device does not match with the usage performance of the inserted developing unit. Therefore, if the developing bias is controlled in accordance with the usage performance of the image forming device, the developing bias does not match with the usage performance of the developing unit. Thus, the normal developing process becomes difficult to be carried out.
In the case of the conventional developing unit, when the remaining amount of the toner in the developing unit becomes low, the developing unit is replaced. However, reusable components such as a frame portion and a roller shaft of the developing unit are reused. With regard to the developing roller arranged in the developing unit, when a replacement timing of the developing roller arrives, the developing roller is replaced with a different developing roller. In this case, when a developing roller having a different developing property is installed, control information relating to the developing bias stored in the image forming device is required to be rewritten according to the newly installed developing roller. As a result, there is a restriction when replacing the developing roller, and the developing unit cannot be recycled efficiently.